A cell culture test system has been developed which indicates the growth-inhibitory properties of coal dust and coal dust extracts. Samples of coal dust and extracts obtained by leaching coal dust in water, calf serum and minimal essential medium have been found to inhibit cell growth. The test system is reproducible and relatively simple. Cells are grown in a medium containing a component exposed to 1% coal dust. The growth of the cells is measured by evaluating the total protein in each culture periodically during 6 days of cultivation. Two coal dusts were tested, one from mines in Pennsylvania, an area in which there is a high incidence of Coal Worker's Pneumoconiosis (CWP), and one from mines in Utah, in which there is a low incidence of the disease. It was found that coal dust from both areas contained extractable substances which inhibited cell growth and this inhibition was much greater in samples from Pennsylvania than in samples from Utah. The results were reproducable and dose dependent. The concentrations of metals extracted from the different coal were determined. Among the metals tested only nickel concentrations correlated with the biological activity. A comparison of the dose response curves for nickel chloride and for coal indicated that nickel may be responsible for much but not all of the cytotoxicity. Chemical fractions are now being tested.